1. Field of the Invention
The present invention relates to a musical tone signal generating apparatus which reads out waveform data from a waveform memory storing waveform data indicative of waveforms of musical tones and generates musical tone signals.
2. Description of the Related Art
As described in Japanese Unexamined Patent Publication No. 9-146555, for example, there is a conventional musical tone signal generating apparatus having a plurality of tone generation channels for generating musical tone signals. The conventional musical tone signal generating apparatus has a waveform memory in which sample values obtained at respective sampling periods by sampling respective musical tones for respective certain tone ranges are compressed to be stored in continuous addresses in the order of sampling period. Because the musical tone signal generating apparatus adopts a compression method of compressing each sample value in relation to the difference between the sample value and the immediately preceding sample value, the musical tone signal generating apparatus is necessary to use the immediately preceding sample value in order to decode compressed data. At each reading of compressed data from the waveform memory, therefore, the musical tone signal generating apparatus increments a reading address by 1.
In the conventional musical tone signal generating apparatus, furthermore, respective periods obtained by time-dividing one period (hereafter referred to as a reproduction period) in which a musical tone signal is generated by respective tone generation channels are assigned as processing periods to the respective tone generation channels. In response to instructions to generate a musical tone signal, more specifically, the respective tone generation channels read compressed data from the waveform memory in their respective assigned processing periods and decode the read compressed data to generate the musical tone signal. In a case where the tone pitch of a sampled musical tone (hereafter referred to as the original pitch) is different from the tone pitch of a musical tone signal which is to be generated (hereafter referred to as the reproduction pitch), sets of compressed data stored in continuous addresses are read out to decode the read compressed data to figure out a plurality of sample values. By performing interpolation by use of the plurality of sample values, the conventional musical tone signal generating apparatus figures out a sample value corresponding to the tone pitch of the musical tone signal which is to be generated.